Handoffs and handoff selection in a wireless access network

ABSTRACT

The present invention facilitates handoffs for a mobile terminal in a wireless access network that is capable of supporting different types of handoffs. The different handoff types may include soft handoffs and fast base station switching (FBSS). In operation, context information associated with supporting wireless communications between the wireless access network and the mobile terminal are determined. Based on whether the context information is shared between base stations involved in the handoff or transferred from one base station to another of the base stations involved in the handoff, a particular handoff type is selected from the different handoff types that are available. Selecting the specific type of handoff to use may also be based on the level of context information that is available, the actual content of the context information, application preferences, channel conditions, base station or mobile terminal capabilities, or any combination thereof.

This application is a National Phase filing based on PCT/IB2005/001571filed Jun. 3, 2005, which claims the benefit of U.S. provisionalapplication Ser. No. 60/577,205 filed Jun. 7, 2004; U.S. provisionalapplication Ser. No. 60/580,906 filed Jun. 21, 2004, and U.S.provisional application Ser. No. 60/598,225 filed Aug. 2, 2004, thedisclosures of which are hereby incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates to wireless communications, and inparticular to various types of handoffs and techniques for selecting anappropriate handoff in a wireless access network.

BACKGROUND OF THE INVENTION

In a wireless access network, numerous base stations are geographicallydistributed in a cellular arrangement and adapted to communicate withvarious mobile terminals. The coverage area, or cells, of the adjacentbase stations generally overlaps. As a mobile terminal moves within agiven cell supported by a base station, or from one cell to another,multiple base stations can support communications with the mobileterminal.

When moving from one cell to another, the wireless access network andthe mobile terminal will cooperate to switch communications from onebase station to another to support continued service and uninterruptedtraffic flow. Such switching is often referred to as a “handoff.” Whenthe base stations are switched during a communication session, integrityof the traffic flow must be maintained.

Switching between base stations generally involves soft or hardhandoffs. Soft handoffs involve multiple supporting base stationssending redundant data during a transition from one base station toanother. Hard handoffs involve a distinct transition from one basestation to another. In another scenario, the service may quickly switchback and forth between multiple base stations, based on channelconditions. Such switching is referred to as either fast base stationswitching (FBSS) or fast cell switching (FCS).

Depending on the access network and the mobile terminal, multiple typesof handoffs may be supported. Accordingly, there is a need for anefficient and effective way to select the most appropriate type ofhandoff to implement in a dynamic fashion. Further, there is a need toimprove the efficiency of the various types of handoffs by providingimproved techniques for implementing the handoffs.

SUMMARY OF THE INVENTION

The present invention facilitates handoffs for a mobile terminal in awireless access network that is capable of supporting different types ofhandoffs. The different handoff types may include soft handoffs and fastbase station switching (FBSS). Different types of soft handoffs as wellas FBSS, such as macro-diversity soft handoffs and selective handoffs,may be used. Further, the FBSS may include fast cell switching (FCS). Inoperation, context information associated with supporting wirelesscommunications between the wireless access network and the mobileterminal are determined. Based on whether the context information isshared between base stations involved in the handoff or transferred fromone base station to another of the base stations involved in thehandoff, a particular handoff type is selected from the differenthandoff types that are available. Selecting the specific type of handoffto use may also be based on the level of context information that isavailable, the actual content of the context information, applicationpreferences, channel conditions, base station or mobile terminalcapabilities, or any combination thereof. When certain levels of contextinformation are not available or the mobile terminal or base stationlacks sufficient capabilities to support a particular type of handoff,hard handoffs may be available, depending on the configuration of thewireless access network.

Those skilled in the art will appreciate the scope of the presentinvention and realize additional aspects thereof after reading thefollowing detailed description of the preferred embodiments inassociation with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the invention, andtogether with the description serve to explain the principles of theinvention.

FIG. 1 is a communication environment according to one embodiment of thepresent invention.

FIGS. 2A and 2B are flow diagrams illustrating network entry in normaloperation according to one embodiment of the present invention.

FIGS. 3A and 3B are a flow diagram illustrating the basic process forselecting a type of handoff according to one embodiment of the presentinvention.

FIGS. 4A-4C are flow diagrams illustrating different hard handofftechniques according to the present invention.

FIGS. 5A-5C illustrate how an active set of base stations is allocatedin a macro-diversity soft handoff embodiment of the present invention.

FIGS. 6A and 6B illustrate macro-diversity soft handoffs according toone embodiment of the present invention.

FIGS. 7A and 7B illustrate antenna assignment in a corresponding softhandoff zone construction according to a first embodiment of the presentinvention.

FIGS. 8A and 8B illustrate antenna assignment in a corresponding softhandoff zone construction according to a second embodiment of thepresent invention.

FIGS. 9A and 9B illustrate antenna assignment in a corresponding softhandoff zone construction according to a third embodiment of the presentinvention.

FIG. 10 illustrates antenna assignment in a corresponding soft handoffzone construction according to a fourth embodiment of the presentinvention.

FIG. 11 is a block representation of a base station according to oneembodiment of the present invention.

FIG. 12 is a block representation of a mobile terminal according to oneembodiment of the present invention.

FIG. 13 is a logical breakdown of a transmitter architecture accordingto one embodiment of the present invention.

FIG. 14 is a block representation of a receiver architecture accordingto one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiments set forth below represent the necessary information toenable those skilled in the art to practice the invention and illustratethe best mode of practicing the invention. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the invention and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure and the accompanying claims.

With reference to FIG. 1, a core communication network 10 is associatedwith a wireless access network (WAN) to facilitate communications with amobile terminal 12. The WAN includes a number of geographicallydistributed base stations 14, which may be associated with a centralnetwork controller 16. The central network controller 16 is a logicalentity, which may be implemented in various nodes or distributed amongmultiple nodes within the WAN. In particular, the central networkcontroller 16 may reside in or be distributed among base stations 14,base station controllers, edge routers, digital subscriber line accessmodems (DSL AMs), or located in one or more nodes in the corecommunication network 10. Logical implementation of a central networkcontroller 16 may also be referred to as a dynamic mobility controlpoint. When distributed, the location of the central network controller16 may change from one location to another depending on the movement ofa particular mobile terminal 12. In one embodiment, each mobile terminal12 is associated with a central network controller 16. Notably, certainembodiments will not require a central network controller 16. CertainWANs may not use a central network controller 16 and will use aspecially configured base station 14, or other type of access point, tointerface with a disparate network, such as the core communicationnetwork 10. The core communication network 10 may be associated withnumerous WANs, and any number of mobile terminals 12 may be within anygiven WAN.

A base station 14 may be any type of wireless access point for cellular,wireless local area network (WLAN), or other wireless communications.During communications, the mobile terminals 12 may move from beingsupported by one base station 14 to another, as well as move from oneWAN to another. The communication coverage provided by each of the basestations 14 is referred to as a cell and may overlap in whole or inpart. As such, the mobile terminal 12 may theoretically be able tocommunicate with multiple base stations 14 at any given time. Thepresent invention addresses selecting an appropriate type of handoff,implementing the selected type of handoff, and controlling traffic flowto and from the mobile terminal 12 through different base stations 14 asservice for the mobile terminal 12 transitions from one base station 14to another during a selected handoff.

Prior to delving into the details of the present invention, differentdata units for carrying any type of information, including audio, video,data, and voice, are defined for clarity. In general, a protocol dataunit (PDU) is a packetized unit of information exchanged over a radiocommunication link between the base station 14 and the mobile terminal12. The PDUs are generally fragmented pieces of higher layer servicedata units (SDUs) In one embodiment of the present invention, an SDU maycorrespond to an Internet Protocol (IP) packet or Ethernet frame. In theWAN, the processing used to convert SDUs into PDUs for downlinkcommunications and PDUs into SDUs for uplink communications may takeplace at the base stations 14, the central network controller 16, or acombination thereof. The mobile terminal 12 will also need to generateSDUs from PDUs during downlink communications and create PDUs from SDUsduring uplink communications. For the wireless link, a PDU is exchangedbetween the media access control (MAC) entities in the mobile terminal12 and the base station 14.

In one embodiment, at least three types of handoffs are potentiallyavailable for use as a mobile terminal 12 moves within a cell or fromone cell to another. For illustrative purposes only, assume there arethree possible handoffs: hard handoffs (HHOs), soft handoffs (SHOs), andfast base station switching (FBSS). The WAN, which may represent one ormore base stations 14, the central network controller 16, or acombination thereof, will interact with the mobile terminal 12 to alloweither the WAN or the mobile terminal 12 to select one of the availablehandoff types based on context information, and perhaps based on whetherthe context information is shared among the base stations 14 involved inthe handoff or transferred from one base station 14 involved in thehandoff to another. The context information generally relates to device,hardware, software, or mobile and network capabilities, which bear onthe availability, desire, or need for a particular type of handoff.

As an example, the context information may be broken into four differentlevels, level 1 through level 4. Notably, context information for eachlevel may include all lower levels. For example, level 3 contextinformation may include level 1 and level 2 context information. Level 1context information may related to hardware or software capabilities ofa mobile terminal 12 or supporting based stations 14, and may relate tocoding rates, types of encoding, physical layer requirements, and thelike. If the hardware and software are configurable, the mobile terminal12 and the base stations 14 involved in the handoff can negotiate andexchange various parameters. Level 2 context information may relate tonetwork access and authentication criteria relating to security andpermission requirements. The level 2 context information may includeidentification information, encryption or decryption keys, passwords, orthe like, and may be provided or exchanged during an authenticationprocess. Level 3 context information may relate to the particularservice or services supported by the WAN, particular base stations 14,and the mobile terminal 12. The service information may identify whetheraudio, video, data, or voice applications are being supported, andwhether the information is being streamed, along with any other servicerelated information. The service related information may include serviceflow identification and quality of service information. Level 4 contextinformation may relate to where and how traffic is controlled in theWAN, and in particular may provide fragmentation control of SDU to PDUconversions, management of active and inactive states, and the abilityto maintain continuity of PDU flows.

For level 3 or level 4 context information, a logical or actual centralnetwork controller 16 may be implemented to provide centralized controlof the various base stations 14 involved in any particular handoff. Insuch an embodiment, the context information may be shared with multiplebase stations 14 at the same time during a handoff. Alternatively, in anembodiment with a central network controller 16 or one without,information may be transferred between base stations 14 during ahandoff. In general, the transferring base station 14 will completelytransfer the context information to another base station 14 to which themobile terminal 12 is being handed off. The base station 14 handing offservice of the mobile terminal 12 will not further use the transferredcontext information. However, the context information may be kept for acertain period of time for future use.

As an example, in configurations where only level 1 or 2 contextinformation is shared or transferred, only a hard handoff is available.In configurations where level 3 or level 4 context information istransferred or shared, the selection of a soft handoff or fast basestation switching may depend on the context information itself orwhether the context information is being transferred or shared. If thelevel 3 or 4 context information is transferred, fast base stationswitching is selected. If the level 3 or level 4 context information isshared, a soft handoff or fast base station switching is available, andthe selection between a soft handoff or fast base station switching isbased on the actual context information, which may be a desire orrequirement of an application associated with a session to be supported,channel conditions, resource availability, or loads on the respectivebase stations 14 involved in the handoff.

The decision to switch between base stations 14 may be based on one ormore criteria in addition to the context information. The criteria mayinclude channel quality information from one or more base stations 14,mobile movement or velocity, the amount of data to be transmitted in theuplink or downlink direction, the existing loads at the various basestations 14, service and traffic flow requirements such as quality ofservice, delay, packet loss, and transfer rate, as well as service type.Those skilled in the art will recognize other criteria that may alone orin combination be used to make decisions regarding control switching.

Depending on the configuration of the WAN and the mobile terminal 12,the context information may be provided to or exchanged between variousentities in different ways and at different times. With reference toFIG. 2A, an exemplary process for allowing a mobile terminal 12 to gainaccess to the WAN is provided. At network entry (step 100), capabilityinformation, including level 1 context information, is negotiated orexchanged between the WAN and the mobile terminal 12 (step 102). Next,an authentication process is provided wherein level 2 contextinformation is provided or exchanged between the WAN and the mobileterminal 12 (step 104). Once authentication is complete, the mobileterminal 12 will register with the WAN (step 106), wherein the mobileterminal 12 may indicate that it can support soft handoffs or fast basestation switching. Next, negotiation or handshaking for establishing aservice connection is provided (step 108), wherein the mobile terminal12 may identify the services for which soft handoffs or fast basestation switching is available. At this point, one can assume that atleast level 3 context information is provided. Further, the exchangesbetween the WAN, the base station 14, and the mobile terminal 12 mayindicate whether transfer or sharing of context information issupported.

With reference to FIG. 2B, normal operation (step 200) of the mobileterminal 12 facilitates uplink or downlink traffic flows for one or morecommunication sessions (step 202). The currently serving base station 14may acquire information from the neighboring base stations 14 (step 204)and broadcast the information received from the neighboring basestations 14 to the supported mobile terminals 12 (step 206). The mobileterminal 12, the base station 14, or a cooperation therebetween, willresult in selecting a particular type of handoff from a number ofavailable types of handoffs (step 208) and initiate a handoff handshake(step 210) to effect a handoff of the selected type (step 212), whereinthe process repeats. Again, the selection of a handoff type may be basedon the actual context information, the level of available contextinformation, or whether the context information is shared or transferredbetween base stations 14 involved in the handoff.

An exemplary handoff selection flow is provided in FIGS. 3A and 3B.Notably, the illustrated process may be implemented in whole or in partin a localized or distributed fashion between the WAN and the mobileterminal 12. The process begins (step 300) by determining whether theWAN supports soft handoffs or fast base station switching (step 302). Ifneither is supported, a hard handoff is used (step 304). If the WANsupports soft handoff and/or fast base station switching, adetermination is made of whether the mobile terminal 12 can support softhandoffs or fast base station switching (step 306). If the mobileterminal 12 does not support either one, a hard handoff is used (step308). If the mobile terminal 12 supports soft handoff and/or fast basestation switching, and assuming level 3 or level 4 context informationis available and sharing or transferring of that information is possible(step 310), a determination is made as to whether the contextinformation will be transferred or shared during the handoff (step 312).If the context information is to be transferred, fast base stationswitching is selected (step 314). If the context information is to beshared, a determination is made as to whether the application for thecommunication session prefers a soft handoff, which is inherently a morerobust handoff technique (step 316). If the application does notnecessarily prefer a soft handoff, fast base station switching is used(step 318). If the application prefers a soft handoff, the interferenceassociated with the communication channel may be used to determinewhether a soft handoff or fast base station switching is used (step320).

If channel quality is in a certain threshold or interference is notabove a defined threshold, fast base station switching is used (step322); otherwise, the relative associated delay with the base stations 14in the active set of base stations 14 is analyzed (step 324). If therelative delay between any base station pair is not less than apredetermined value, then fast base station switching is used (step326); otherwise, the decision between soft handoff and fast base stationswitching is decided based on whether a soft handoff zone is defined(step 328). If a soft handoff zone is not defined, then fast basestation switching is used (step 330). If a soft handoff zone is defined,then a soft handoff is used (step 332). Those skilled in the art willrecognize alternative decision-making processes for selecting betweensoft handoffs and fast base station switching, as well as other types ofhandoffs, in light of the concepts described and exemplified herein.

Turning now to FIG. 4A through FIG. 10, various handoff techniques aredescribed. With particular reference to FIG. 4A, a hard handoff scenariois illustrated, wherein no context information is available or onlylevel 1 information is shared or transferred between base stations 14involved in the handoff. Initially, the serving base station 14 willbroadcast information pertaining to neighboring base stations 14 to themobile terminal 12 (step 400). The information may identify the physicalparameters of the neighboring base stations 14, their acceptablecarrier-to-interference ratio thresholds, and available or supportablebandwidth. The mobile terminal 12 will determine which of the basestations 14 to scan, or monitor, based on the broadcast information(step 402). The mobile terminal 12 may initiate a conditional scan of aset of selected base stations 14 (step 404). In one embodiment, themobile terminal 12 will request a scan only if there is no uplink datapending, or the base station 14 will only confirm a scan if there is nodownlink data pending. The mobile terminal 12 will then select a targetbase station 14 to which a handoff is desired (step 406). A base station14 may be selected as a target base station 14 if the measuredcarrier-to-interference ratio associated with a target base station 14is higher than the predefined acceptable carrier-to-interferencethreshold and the additional supportable bandwidth is sufficient for thecommunication session or otherwise acceptable by the mobile terminal 12.

Next, the base station 14 and the mobile terminal 12 will initiateconditional pre-ranging to the target base station 14 (step 408). In aneffort to minimize the impact on existing sessions, the conditionalpre-ranging step may be provided only when there is no data pending inboth the downlink and uplink traffic flows, and no immediate handoff isneeded. When pre-ranging is initiated, the serving base station 14 willinform the target base station 14 that pre-ranging is requested. Thetarget base station 14 will respond with an assigned ranging resource,which may include a temporary handoff identification. The serving basestation 14 will relay the information, which may include a temporaryhandoff identification, to the mobile terminal 12. Regardless of whetherpre-ranging is initiated, the base station 14 and the mobile terminal 12will initiate a pre-handoff process, wherein the serving base station 14may or may not transfer corresponding context information to the targetbase station 14 (step 410). The target base station 14 may assign aranging resource, as well as connection identifications for the existingsessions.

At this point, a switch is made from the serving base station 14 to thetarget base station 14. If the handoff does not occur within apredefined time window after pre-ranging, the mobile terminal 12 mayimplement conditional initial ranging (step 412). The initial rangingallows the mobile terminal 12 to obtain the connection IDs for thesession flows from the newly serving (target) base station 14, which mayinteract with the originally serving base station 14 through the WAN toobtain the information. The mobile terminal 12 will then initiate are-authentication process (step 414), as well as a re-registrationprocess, and then re-establish session flow (step 416). At this point,the base station 14 and the mobile terminal 12 will begin normaloperation (step 418).

From the above, several observations may be made. When the serving basestation 14 broadcasts its neighbors' information to the mobile terminal12, information such as available bandwidth and an acceptablecarrier-to-interference ratio threshold for the handoff may be added tothe information. Such additional information will help the mobileterminal 12 determine which of the neighboring base stations 14 to scanin order to avoid an unnecessary scan of neighboring base stations 14that cannot support a session requiring a certain bandwidth, quality ofservice, or would otherwise be overloaded if the session were takenover. These scans may be conditional and may only happen if there is nodata pending transfer on either the downlink or uplink session flows.Pre-ranging may be conditional and performed only if there is a smallamount of data to be transferred. If pre-ranging is not performed, themobile terminal 12 will take the necessary steps to speed up initialranging with the target base station 14 after a handoff is completed.

If pre-ranging is performed, the target base station 14 may first assigna dedicated uplink resource and a temporary handoff identification tothe mobile terminal 12. The serving base station 14 may then relay thoseassignments to the mobile terminal 12 through control signaling. Suchsignaling may be a fast ranging information element used in the IEEE802.16e. This action may avoid delay in ranging when the mobile terminal12 waits for the fast ranging information element. The mobile terminal12 may then use these resources to perform pre-ranging, wherein atemporary handoff identification may be used. The initial ranging to thetarget base station 14 may be conditional. If the handoff happens withina pre-defined time window after pre-ranging, initial ranging may bebypassed. During the handoff procedure, a target base station 14 canassign a basic connection ID, primary or secondary connection IDs, andmaybe even flow connection IDs. The serving base station 14 may sendsuch information to the mobile terminal 12 before handoff to avoid usingrenegotiation techniques to assign connection IDs, if an initial rangingto the target base station 14 can be bypassed.

With reference to FIG. 4B, another hard handoff scenario is illustrated.The first five steps (steps 500-508) are the same as those illustratedin FIG. 4A (steps 400-408). This process differs in that the pre-handoffprocess (step 510) involves transferring level 2 context information. Iflevel 2 context information is not shared, the serving base station 14may transfer the level 2 context information to the target base station14. The target base station 14 will then assign a dedicated uplinkresource for initial ranging of the mobile terminal 12 and for obtainingthe connection IDs. The target base station 14 will send thisinformation to the serving base station 14, which will relay theinformation to the mobile terminal 12. Since the level 2 informationrelates to authentication, the re-authentication step (step 414 of FIG.4A) is eliminated after switching to the target base station 14. Assuch, the mobile terminal 12 may initiate conditional initial ranging(step 512), as well as initiate re-registration and re-establish sessionflow (step 514). At this point, normal operation between the basestation (target) 14 and the mobile terminal 12 begins (step 516).

With reference to FIG. 4C, again the first five steps of the process(steps 600-608) correspond to those steps (400-408) described inassociation with FIG. 4A. For the pre-handoff process (step 610), whenlevel 3 context information is not shared, the serving base station maytransfer the level 3 context information to the target base station 14.The target base station 14 will assign a dedicated uplink resource forthe initial ranging of the mobile terminal 12 and the connection IDs.The target base station 14 will forward this information to the servingbase station 14, which will relay it to the mobile terminal 12. Notably,transferring context information at level 3 implies that lower levelcontext information, such as level 1 and level 2 context information, isincluded in the level 3 context information. As such, the level 3context information will include information pertaining toauthentication, registration, and control of session flows. As such, thesteps of re-authentication, re-registration, and re-establishment ofsession flows are eliminated. After switching to the target base station14, the mobile terminal 12 may initiate conditional initial ranging, ifnecessary (step 612) and then cooperate with the serving (target) basestation 14 to begin normal operation (step 614).

As noted above, soft handoffs may be one of the types of handoffsavailable to the WAN and the mobile terminal 12. The soft handoffs maybe implemented in different ways, as those skilled in the art willappreciate. One particular handoff that may be implemented is onereferred to as “macro-diversity” soft handoff. The macro-diversity softhandoff has the following characteristics. For downlink communications,multiple base stations 14 will transmit the same PDUs to the targetmobile terminal 12. The transmission of corresponding PDUs from thevarious base stations 14 will occur within a predefined period of time,wherein the mobile terminal 12 will use a soft combining process fordecoding the transmitted PDUs, preferably prior to any forward errorcorrection decoding. For uplink communications, transmission of the PDUsfrom a given mobile terminal 12 may be received and decoded by multiplebase stations 14. The decoded PDUs from the various base stations 14 arethen processed to recover the actual PDUs transmitted.

The set of base stations 14 involved in the macro-diversity soft handoffwith a given mobile terminal 12 is referred to as the active set of basestations 14 for the mobile terminal 12. Different mobile terminals 12may have different active sets of base stations 14. The active set ofbase stations 14 may be established by the mobile terminal 12, andmaintained in the central network controller 16 or other desiredlocation. Further, the active set of base stations 14 can be modified bythe mobile terminal 12 as well as by the base stations 14.

During operation, each mobile terminal 12 will consistently measure allor a subset of the base stations 14 advertised by the WAN, including thesignal strengths and the relative propagation delays based on thecell-specific pseudo noise (PN) codes transmitted in the preambles. Thebase stations 14 may also provide cell identification informationthrough these advertisements. The mobile terminal 12 may identify thecorrelation peaks for all or a subset of the base stations 14, and therelative delays associated with the base stations 14. The mobileterminal 12 will then decide to add a base station 14 into the activeset of base stations 14, if certain or all of the following criteria aremet. First, if the signal strength, or correlation peak, associated withthe base station 14 exceeds a certain predefined absolute threshold orrelative threshold compared to a base station 14 associated with thebest signal strength. Second, if the relative delay of the correlationpeak for the associated base station 14, with respect to the best basestation 14, falls within a prefix interval. If the delay is beyond apredetermined value, a hard handoff or fast base station selection canbe applied. The mobile terminal 12 can adjust its timing according tothe current timing and the relative delay between the serving basestation 14 and the previously serving base station 14 during the handoffto help maintain time references for the relative delays.

Once the mobile terminal 12 identifies the candidate base stations 14 tobe added to the active set of base stations 14, the mobile terminal 12may send a message containing the signal strength and delay informationof the candidate base stations 14 to the serving base station 14. Thisinformation may be sent over an uplink communication channel orsub-carrier in a layer 2 message. Based on the information sent by themobile terminal 12 to the serving base station 14, the active set ofbase stations 14 may decide which of the candidate base stations 14 toadd to the active set. The base stations 14 in the mobile terminal'scurrent active set of base stations 14 may send an update messageidentifying the new active set of base stations 14 to the mobileterminal 12. The message may include an action time, at which both themobile terminal 12 and the affected base stations 14 will update theactive set of base stations 14 and operate using this active set of basestations 14.

Under the specific context of 802.16e, a mobile terminal 12 canestablish parallel uplink and downlink sessions with multiple basestations 14 at the same time. These sessions may be connections orflows. In one embodiment, to support soft handoff in this scenario, theactive set of base stations 14 will share level 3 or level 4 contextinformation. For a given session, the mobile terminal 12 can establish acommon connection identifier with multiple ones of the base stations 14or a different connection identifier with each of the affected basestations 14. When different connection IDs are used, the connection IDsare uniquely mapped to the given session, and the mobile terminal 12will know which connection IDs correspond to which base stationconnections for the session. The connection ID corresponding to eachsession is set up when the mobile terminal 12 adds a base station 14into its active set of base stations 14.

In one embodiment, the downlink control signaling is sent via theserving base station 14 to the mobile terminal 12 when macro-diversitysoft handoff is implemented. Alternatively, a new downlink controlsub-channel can be defined and allocated to the mobile terminal 12. Thecontrol signaling on the sub-channel may be sent by all of the basestations 14 in the mobile terminal's active set of base stations 14.

The uplink feedback signaling may include the existing fast feedbackchannel, and a newly introduced cell switching channel. The uplinkfeedback signaling and other uplink control signaling can be received ineither soft handoff or non-soft handoff modes. In a non-soft handoffmode, the uplink feedback and control signaling is received by theserving base station 14. The serving base station 14 may update theshared macro-diversity management context information among theappropriate base stations 14. In a soft handoff mode, the uplinkfeedback and control signaling is received by all of the base stations14 in the mobile terminal's active set of base stations 14. Theinformation decoded by each of the base stations 14 is effectivelycombined to recover the originally transmitted information and isprocessed accordingly. When multicasting from the active set of basestations 14, upper layer packets, such as the SDUs, are made availableto the active set of base stations 14 that share the samemacro-diversity context information. The active set of base stations 14transmits the same set of PDUs, which are derived from the SDUs, to themobile terminal 12. These transmissions will occur during a predefinedwindow and will be soft combined at the mobile terminal 12 prior todecoding to recover the originally transmitted data. In an orthogonalfrequency division multiple access (OFDM) system, the PDUs are sent in adownlink OFDM zone reserved for soft handoffs among the base stations 14in the active set. The zone is generally defined as sub-carriers.

With reference to FIGS. 5A-5C, FIG. 5A illustrates a sectorized,three-cell communication environment and FIG. 5B is a graph illustratingthe active set of base stations 14 for each mobile terminal 12. Forclarity, the mobile terminals 12 are referenced as mobile terminals A-Oand the base stations 14 are represented as base stations 1-9. Thedarkened boxes in each row represent the active set of base stations 14for a given mobile terminal 12. For example, mobile terminal M has basestations 2, 8, and 9 in its active set of base stations 14. FIG. 5Cillustrates a scheduling table for macro-diversity soft handoff. Thetable illustrates communication times, referenced as frame numbers,between the base stations 1-9 and the mobile terminals A-O. Asillustrated, during frames 1 and 6, base stations 3, 4, and 5 willcommunicate with mobile terminal C, and so on.

With reference to FIG. 6A, the three highlighted cells represent acluster of base stations 14 within a distance corresponding to a prefixlength, wherein the prefix is a prefix associated with a PDU. Thecluster of base stations 14 may be defined when the WAN is deployed andthe prefix length can be configured by the WAN. With reference to FIG.6B, the sub-channel permutation in a soft handoff zone may be arrangedas follows to achieve interference avoidance:

{(CellID)mod 2} for partition-B

{(CellID)mod 3} for partition-C

Turning now to FIGS. 7A and 7B, antenna assignment for communications ina macro-diversity soft handoff is addressed. As illustrated in FIG. 7A,two base stations 14, BS-1 and BS-2 are illustrated having antennas {α,1} and {β, 1} as well as {α, 2} and {β, 2}, respectively. For a singleantenna, the two base stations BS-1 and BS-2 can use the followingtransmit formats: space-time transmit diversity (STTD), BLAST, andcyclic delay diversity (CDD).

For the multiple-input, multiple-output (MIMO) antenna configurationillustrated in FIG. 7A, an OFDM embodiment may have the sub-carriers (orsub-channels) broken into m sub-blocks as shown in FIG. 7B. Eachsub-block will consist of two sub-channels, wherein the STTD and BLASTtransmit formats can be used to implement space-time coding (STC). STCPair-1 and STC Pair-2 are illustrated in FIGS. 7A and 7B.

With reference to FIGS. 8A and 8B, a three-base station configuration isillustrated to include base stations BS-1, BS-2, and BS-3. Base stationBS-3 includes two antennas {α, 3} and {β, 3}. When using a singletransmit antenna, the three base stations BS-1, BS-2, and BS-3 can use aCDD-BLAST transmit format. For MIMO antenna configurations, the softhandoff zone may consist of n sub-blocks, each consisting of threesub-channels wherein STTD and BLAST transmit formats can be used.

FIGS. 9A and 9B illustrate embodiments where the channel qualityindicator channel can be used by the active base station 14 for antennaassignment and weighting (W₁-W₄) of the respective signals (S₁ and S₂).The embodiment of FIG. 9A may support a two-bit feedback for thefour-antenna pairing configurations (sub-MIMO). FIG. 9B presents anembodiment where the channel quality indicator channel is used to feedback four antenna weights to facilitate beamforming for transmissions tothe mobile terminal 12 or other base stations 14.

For base stations 14 capable of performing uplink MIMO communications,the soft handoff zone can be overlaid atop a conventional soft handoffzone. As illustrated in FIG. 10, to reduce the interference of theregular traffic on the soft handoff traffic, as well as improvedetection performance during movement, the visual MIMO soft handoffpartner should be chosen from the mobile terminals 12 more proximate tothe base stations BS-1 and BS-2.

For fast base station switching, an active set of base stations 14 isdefined for each mobile terminal 12 supporting fast base stationswitching. The active set of base stations 14 is established based onthe procedure described for soft handoffs. The mobile terminal 12 willactively switch among the base stations 14 in the active set of basestations 14, generally based on channel conditions. Preferably, theserving base station 14 is the one in the active set of base stations 14that has the strongest downlink performance. Fast switching from theserving base station 14 to a target base station 14 may be triggeredwhen the mobile terminal 12 detects a signal strength of a non-servingbase station 14 as being higher than that of the currently serving basestation 14 by a defined ratio, threshold, or other criteria. The mobileterminal 12 may send a Layer 2 message to the active set of basestations 14 to identify the target base station 14.

Alternatively, the mobile terminal 12 may send an indication on acontrol channel, referred to as a cell switch indication channel(CSICH). The CSICH may contain three bits of information to carry thecompressed base station ID. The compressed base station ID is an IDassigned to a base station 14 when the base station 14 is added to themobile terminal's active set of base stations 14. The base station ID isincluded in the active set update message, which is sent by the basestation 14 to the mobile terminal 12. The switching will generally takeplace at a predefined time interval after the mobile terminal 12 sendsthe Layer 2 switching message or the CSICH. Alternatively, the basestation 14 can send a cell switching direction Layer 2 message on thedownlink channel to the mobile terminal 12 with an action time.

With regard to ranging, no ranging is needed before switching when themobile terminal 12 is switching to a base station 14 in the active setof base stations 14. For traffic flow continuity, the serving basestation 14 can transfer the mobile terminal's PDUs, which may be Layer 2frames, and Layer 2 automatic receipt request (ARQ) status to the targetbase station 14 when the traffic data and level 4 context information ofthe mobile terminal 12 are not shared by both the serving and targetbase stations 14. If level 4 context information is not shared by theserving and target base stations 14, the traffic data can be madeavailable to both the serving and target base stations 14. In this case,the serving base station 14 only needs to inform the target base station14 of the PDUs that the target base station 14 should start transmittingto the mobile terminal 12.

From the above, fast base station switching is a feature that allows themobile terminal 12 to quickly switch from one base station 14 to anotherwithin the active set of base stations 14. To support fast base stationswitching, the present invention provides a mechanism for the mobileterminal 12 to use to report the newly selected anchor (target) basestation 14 and a way to synchronize when actual switching to the newanchor base station 14 occurs. In a first scenario, the mobile terminal12 uses a dedicated uplink channel to report the selected anchor basestation 14. Accordingly, the mobile terminal 12 reports the selection ofan anchor base station 14 using one or more dedicated anchor basestation selection (ABSS) feedback channels during uplink communications.The ABSS feedback channel or channels are allocated to the mobileterminal 12 by the base station 14. The mobile terminal's operation isas follows.

Assuming the mobile terminal's current anchor base station 14 is basestation A, assume mobile terminal 12 decides to select base station B asthe new anchor base station. The mobile terminal 12 will send theselection information on the ABSS feedback channel and then start atimer for a transition period. If the mobile terminal 12 does notreceive any downlink control signaling from base station A indicatingcanceling of a switching operation to base station B, or indicating atarget anchor base station C during the action time for switching beforethe switch indication retransmission timer times out, the mobileterminal 12 will switch to anchor base station B when the timer timesout. If the mobile terminal 12 receives information indicating theswitching operation to base station B should be cancelled before theswitch indication retransmission timer times out, the mobile terminal 12will send an acknowledgement message to acknowledge the receipt of theinformation. The mobile terminal 12 will then cancel the switchingoperation and disable the switch indication retransmission timer.

If the mobile terminal 12 receives information indicating a targetanchor base station C and an action time for switching before the switchtransition period times out, the mobile terminal 12 will send anacknowledgement message to acknowledge receipt of the information. Themobile terminal 12 will then reset the switch indication retransmissiontimer to the action time and switch to the target anchor base station Cwhen the switch indication retransmission timer times out. After themobile terminal 12 sends the selection information on the ABSS feedbackchannel to indicate a newly selected anchor base station B, the mobileterminal 12 will not send selection information indicating another newanchor base station C before the switch indication retransmission timertimes out. However, the mobile terminal 12 may send the selectioninformation on the ABSS feedback channel indicating the selection ofbase station A as the anchor base station 14. It is then up to theserving base station A to decide whether to send information to cancelthe switch to base station B.

If base station A receives the selection information on the ABSSfeedback channel from the mobile terminal 12 indicating newly selectedanchor base station B, base station A may choose to send information tothe mobile terminal 12 before the switch indication retransmission timertimes out. The information may cause the mobile terminal 12 to cancelthe switching operation to base station B, or to switch to a particulartarget anchor base station 14 at a specific action time.

In another scenario, the mobile terminal 12 may use a machine accesscontrol (MAC) header or sub-header to report the selected anchor basestation 14. The mobile terminal 12 may report the selection of an anchorbase station 14 using one or more ABSS feedback channels on the uplink.The ABSS feedback channels are allocated to the mobile terminal 12 bythe base station 14. Assuming the current anchor base station 14 is basestation A, the mobile terminal 12 may decide to select base station B asthe new anchor base station 14. The mobile terminal 12 will send theselection information in the mode selection feedback MAC header when nouplink traffic is present, or in a sub-header when uplink traffic ispresent. The mobile terminal 12 will then start a timer, referred to asa switch indication retransmission timer. If the mobile terminal 12 doesnot receive any downlink control signaling from base station Aindicating that switching operation to base station B should becancelled, or indicating a target anchor base station C and an actiontime for switching, before the switch indication retransmission timertimes out, the mobile terminal 12 will resend the mode selectionfeedback MAC header or sub-header, and restart the switch indicationretransmission timer. If the mobile terminal 12 receives informationindicating canceling of the switching operation to base station B beforethe switch indication retransmission timer times out, the mobileterminal 12 will send an acknowledgement message to acknowledge receiptof the information. The mobile terminal 12 may cancel the switchingoperation and disable the switch indication retransmission timer. If themobile terminal 12 receives information indicating a target anchor basestation C and an action time for switching before the switch indicationretransmission timer times out, the mobile terminal 12 will send anacknowledgment message to acknowledge receipt of the information. Themobile terminal 12 will then set the switch transmission period timer tothe action time, and will switch the target anchor base station C whenthe switch indication retransmission timer times out.

After the mobile terminal 12 sends the selection information on the ABSSfeedback channel indicating newly selected anchor base station B, themobile terminal 12 may not send selection information indicating anothernew anchor base station 14 before the switch indication retransmissiontimer times out. However, the mobile terminal 12 may send the selectioninformation on the ABSS feedback channel indicating selection of basestation A as the anchor base station 14. It is then up to base station Ato decide whether to send information to cancel the switch to basestation B.

If base station A receives selection information on the MAC header orsub-header from the mobile terminal 12 identifying newly selected anchorbase station B, base station A will send information to the mobileterminal 12 before the switch indication retransmission timer times outat the mobile terminal 12. The information will trigger the mobileterminal 12 to cancel the switch to base station B, or cause the mobileterminal 12 to switch to a particular target anchor base station 14 at aspecific action time.

The information referenced above may be provided in an anchor basestation switch information element. When the base station 14 sends ananchor base station switch information element to the mobile terminal12, the base station 14 will at the same time allocate uplink channelresources to the mobile terminal 12 to send an acknowledgement of thereceipt of the anchor base station switch information element. After themobile terminal 12 receives the anchor base station switch informationelement, the mobile terminal 12 will send an acknowledgement on theuplink channel resource allocated by the base station 14. In oneembodiment of the invention, the acknowledgement is carried by the MACheader, if no concurrent uplink burst allocation is present, or a MACsub-header, if concurrent uplink burst allocation is present. The basestation 14 will allocate sufficient uplink channel resources for themobile terminal 12 to send the MAC header or sub-header. In anotherembodiment, the format of the MAC header and sub-header are in the formof the mode selection feedback MAC header and sub-header, respectively.

In one embodiment, the mobile terminal 12 may operate in a sleep mode.While in the sleep mode, the mobile terminal 12 is unavailable to theserving base station 14 for uplink and downlink traffic. The mobileterminal's sleep interval consists of sleep windows interleaved withlistening windows. During sleep windows, the mobile terminal 12 does notreceive any control information and does not receive or transmit anyuplink or downlink traffic. During listening windows, the mobileterminal 12 listens to downlink broadcasts of synchronizationinformation, control signaling, and broadcast MAC messages. The presentinvention also provides a mechanism to support soft handoff and fastbase station switching for mobile terminals 12 in sleep mode.

For normal mode, an active set of base stations 14 is maintained for themobile terminal 12 while operating in soft handoff or fast base stationswitching. For sleep mode, an active set of base stations 14 is alsomaintained. While in sleep mode, the mobile terminal 12 will continue tomeasure the signal strength of serving and neighboring base stations 14.During the sleep window, if the mobile terminal 12 detects that aneighbor base station 14 not currently in the active set has a signalstrength higher than a specific threshold, or if the mobile terminal 12detects that a base station 14 currently in the active set has a signalstrength lower than a specific threshold, the mobile terminal 12 willwake and maintain synchronization for uplink and downlink communicationson the current anchor base station 14. Once synchronized, the mobileterminal 12 will initiate updating of the active set of base stations14. Once the active set of base stations 14 is updated, and if themobile terminal 12 is still within the sleep window, the mobile terminal12 will go back into sleep mode. If the mobile terminal 12 is within thelistening window after the actives set of base stations 14 is updated,the mobile terminal 12 will remain in the listening mode.

During the listening window, the mobile terminal 12 will synchronizeuplink and downlink communications with the anchor base station 14. Ifthe mobile terminal 12 detects that a neighbor base station 14 that isnot currently in the active set has a signal strength higher than aspecific threshold or if the mobile terminal 12 detects that a basestation 14 currently in the active set has a signal strength lower thana specific threshold, the mobile terminal 12 will update the active set.Once the active set of base stations 14 is updated and if the mobileterminal 12 is still within the listening window, the mobile terminal 12will remain in listening mode. If the mobile terminal 12 is within thesleep window after the active set of base stations 14 is updated, themobile terminal 12 will remain in sleep mode.

For normal mode, fast anchor base station selection information is sentto the mobile terminal 12 on either a dedicated uplink channel allocatedby the base station 14, or on a mode selection feedback MAC header orsub-header. For the case of sleep mode, the mobile terminal 12 will notbe assigned a dedicated uplink channel, such as the ABSS feedbackchannel, since this will cause unnecessary overhead. The mobile terminal12 will instead use the mode selection feedback MAC header to sendanchor base station selection information. While in a sleep mode, themobile terminal 12 will continue to measure the signal strength of theserving and neighboring base stations 14. During the sleep window, ifthe mobile terminal 12 decides to select a new anchor base station 14based on various criteria, such as signal strength, the mobile terminal12 will awake and synchronize to the current anchor base station 14.Once synchronized, the mobile terminal 12 will initiate fast anchor basestation updating by sending a bandwidth request ranging code. Once thebase station 14 detects the bandwidth request ranging code, the basestation 14 will allocate an uplink resource for the mobile terminal 12to send the mode selection feedback MAC header. Once the mobile terminal12 receives the uplink resource allocation, the mobile terminal 12 willsend the mode selection feedback MAC header to carry the information onthe anchor base station selection. The anchor base station updateoperation continues as in the case of normal mode. Once the anchor basestation update operation is completed, and if the mobile terminal 12 isstill within the sleep window, the mobile terminal 12 will go back tosleep mode. If the mobile terminal is within the listening window afterthe anchor base station update operation is completed, the mobileterminal 12 will remain in listening mode.

During the listening window, if the mobile terminal 12 decides to selecta new anchor base station 14 based on a factor such as signal strength,the mobile terminal 12 will synchronize with the current anchor basestation 14. Once synchronized, the mobile terminal 12 will initiate fastanchor base station updating by sending a bandwidth request rangingcode. Once the base station 14 detects the bandwidth request rangingcode, the base station 14 will allocate uplink resources for the mobileterminal 12 to send the mode selection feedback MAC header. Once themobile terminal 12 receives the uplink resource allocation, the mobileterminal 12 will send the mode selection feedback MAC header to carrythe information on the new anchor base station selection. The anchorbase station update operation continues as in the case of normal mode.Once the anchor base station update operation is completed, and if themobile terminal 12 is still within the listening window, the mobileterminal 12 will remain in listening mode. If the mobile terminal 12 iswithin the sleep window after the anchor base station update operationis completed, the mobile terminal 12 will remain in sleep mode.

As noted, there are several types of soft handoffs, some of which use anactive set of base stations 14 of base stations 14. With macro-diversitysoft handoff, the active set of base stations 14 may transmit in exactlythe same time, frequency window. For example, for OFDM, there is afrequency dimension consisting of sub-carriers, and a time dimensionconsisting of OFDM symbols. With selective soft handoff, different basestations 14 may transmit with different windows in the time/frequencyplane. Then a receiver can detect multiple signals separately and make aselection between them. With fast base station switching, the mobileterminal 12 only communicates with a single base station 14 at a time.This does not require a synchronized CID. A re-negotiation can takeplace each time there is a switch in base stations 14

Typically, an anchor base station 14 is identified for the purpose ofdownlink signaling. The mobile terminal 12 can receive multiple downlinktraffic channels, but has a dedicated control channel from the anchorbase station 14 for signaling.

Certain embodiments of the invention provide solutions to CIDmanagement, active set establishment and management, anchor base station14 switching, UL feedback channel (CQICH) switching, UL feedback channelreporting, and UL data transmission for each of the above introducedsoft handoff schemes. These are particularly suited to, but not limitedto, implementations of the 802.16e standard.

CID Management

In macro-diversity soft handoff implementations, for selective softhandoff implementations, and for MIMO BLAST implementations, accordingto select embodiments of the invention, all members of the active set ofbase stations 14 assign the same CIDs, and the CIDs are then updated ifneeded when any new base station(s) 14 is added to active set of basestations 14. For example, if base station 14A and base station 14B arein the active set of base stations 14, and CID0000 is being used, whenbase station 14C is added, it may be necessary to update the CID so thatall three base stations 14A-14C can use the same CID.

With fast base station switching implementations, when the anchor basestation 14 is switched, the CID is updated if needed. There is no needto synchronize CIDs among base station 14s in an active set of basestations 14 for such implementations.

Active Set Establishment and Management

For macro-diversity soft handoff implementations, preferably a set ofmessages are established to allow management and establishment of theactive set of base stations 14:

-   -   MOB_MSsoft handoff_REQ (mobile subscriber station handoff        request) is a message for allowing a mobile terminal 12 to        request soft handoff or request a change in members of the        active set of base stations 14. The mobile terminal 12 monitors        neighbors and reports signal strengths, and identifies the base        stations 14 that it wants to be part of the active set of base        stations 14.    -   MOB_Bsoft handoff_REP (base station handoff response) is a        message in response to the above request in which the base        station 14 indicates which base stations 14 should be in the        active set of base stations 14. Preferably, a flag will be        employed to indicate which base station 14 is the anchor base        station 14.    -   MOB_HO_IND is a message that confirms the update of the active        set of base stations 14.    -   MOB_Bsoft handoff_REQ is a message for the base station 14 to        initiate a handoff. It contains a recommended active set of base        stations 14.    -   MOB_MSsoft handoff_RSP is a response from the mobile terminal 12        to the base station initiated handoff.

For selective soft handoff implementations, preferably a set of messagesis established to allow management and establishment of the active setof base stations 14:

-   -   MOB_MSsoft handoff_REQ (mobile subscriber station handoff        request) is a message for allowing a mobile terminal 12 to        request soft handoff or request a change in members of the        active set of base stations 14. The mobile terminal 12 monitors        neighbors and reports signal strengths, and identifies the base        stations that it wants to be part of the active set of base        stations 14.    -   MOB_Bsoft handoff_REP (base station handoff response) is a        message in response to the above request in which the base        station 14 indicates which base station(s) 14 should be in the        active set of base stations 14. Preferably, a flag will be        employed to indicate which base station 14 is the anchor base        station 14.    -   MOB_HO_IND is a message that confirms the active set of base        stations 14 update.    -   MOB_Bsoft handoff_REQ is a message for the base station 14 to        initiate a handoff. It contains a recommended active set of base        stations 14.    -   MOB_MSsoft handoff_RSP is a response from the mobile terminal 12        to the base station initiated handoff.

For Fast cell switching implementations, preferably a set of messagesare established to allow management and establishment of the active setof base stations 14:

-   -   MOB_MSsoft handoff_REQ (mobile subscriber station handoff        request) is a message for allowing a mobile terminal 12 to        request soft handoff or request a change in members of the        active set of base stations 14. The mobile terminal 12 monitors        neighbors and reports signal strengths, and identifies the base        stations 14 that it wants to be part of the active set of base        stations 14.    -   MOB_Bsoft handoff_REP (base station handoff response) is a        message in response to the above request in which the base        station 14 indicates which base stations 14 should be in the        active set of base stations 14. Preferably, a flag will be        employed to indicate which base station 14 is the anchor base        station 14.    -   MOB_HO_IND is a message that confirms the update of the active        set of base stations 14.    -   MOB_Bsoft handoff_REQ is a message for the base station 14 to        initiate a handoff. It contains a recommended active set of base        stations 14.    -   MOB_MSsoft handoff_RSP is a response from the mobile terminal 12        to the base station initiated handoff.

For MIMO Blast implementations, preferably a set of messages areestablished to allow management and establishment of the active set ofbase stations 14:

-   -   MOB_MSsoft handoff_REQ (mobile subscriber station handoff        request) is a message for allowing a mobile terminal 12 to        request soft handoff or request a change in members of the        active set of base stations 14. The mobile terminal 12 monitors        neighbors and reports signal strengths, and identifies the base        stations 14 that it wants to be part of the active set of base        stations 14.    -   MOB_Bsoft handoff_REP (base station handoff response) is a        message in response to the above request in which the BTS        indicates which base stations 14 should be in the active set of        base stations 14. Preferably, a flag will be employed to        indicate which base station 14 is the anchor base station 14.    -   MOB_HO_IND is a message that confirms the update of the active        set of base stations 14.    -   MOB_Bsoft handoff_REQ is a message for the base station 14 to        initiate a handoff. It contains a recommended active set of base        stations 14.    -   MOB_MSsoft handoff_RSP is a response from the mobile terminal 12        to the base station initiated handoff.        Anchor Base Station 14 Switching

For Macro-diversity soft handoff, a number of anchor base station 14switching options are provided:

Option 1—Message Handshake:

-   -   mobile terminal 12 initiated—use the above introduced active set        update messages (MOB_MSsoft handoff-REQ/MOB_Bsoft        handoff_RSP/MOB_HO_IND), but include an additional field to        indicate the anchor base station.    -   base station 14 initiated use the above introduced Active set        update messages (MOB-Bsoft handoff-REQ/MOB_HO_IND), but include        an additional field to indicate the anchor base station.

Option 2—Define a New DL MAP IE (Anchor Switching Information Element)to Indicate Anchor Switching

-   -   mobile terminal 12 initiated: MOB_MSsoft handoff-REQ/anchor        switching IE    -   base station 14 initiated: MOB_Bsoft handoff-REQ/MOB_MSsoft        handoff-RSP/anchor base station 14 switching IE

Option 3—Define a New Feedback Reporting IE and Anchor Switching IE.

-   -   Mobile terminal initiated: Feedback_reporting_IE (sent by base        station 14)/Anchor base station 14 indication (sent by mobile        terminal 12 over feedback channel)/anchor_switching IE. With        this solution, in some embodiments, the feedback_reporting_IE is        used for both feeding bask C/I information and for sending an        anchor base station identifier. Some indication needs to be        generated either at the base station 14 or mobile terminal 12 to        convey that a given feedback_reporting_IE will contain the        anchor base station identifier instead of the        carrier-to-interference ratio information.

Option 4—Monitor Multiple Base Station 14s

-   -   Same as above 3 options with one exception that the mobile        terminal 12 monitors the MAPs (resource allocations—might for        example have time and sub-carrier information for OFDM        applications but may be different for different physical layers)        from all member base stations 14 to know when and to which base        station 14 to switch. The base station 14 that sends the new MAP        is the new anchor.

Option 5—Define New Dedicated Anchor Base Station 14 Switching Channelfor a Mobile Terminal 12 that Supports Soft Handoff or Fast Base StationSwitching

-   -   For selective soft handoff, the options presented above for        macro-diversity soft handoff are also provided.

For fast base station switching, again a number of implementations areprovided.

Option 1—Message Handshake

-   -   Same as macro-diversity soft handoff

Option 2—Using Feedback Channel and Other BS IE as a Pointer (EmptyResource)

-   -   Mobile terminal 12 initiated: periodic indication over Feedback        channel/Other_BS_IE    -   base station 14 initiated: Other_BS_IE

Option 3—Define New Feedback Reporting IE to Indicated Reporting Contentat the Next Transmission of Feedback Channel

-   -   Mobile terminal 12 initiated: feedback_reporting_IE (base        station 14 sent)/base station 14 switching indication (mobile        terminal 12 sent over feedback channel)/Other_BS_IE

Option 4—Define New Feedback Reporting IE and Anchor Switching IE

-   -   Mobile terminal 12 initiated: Feedback_reporting_IE (sent by        base station 14)/Anchor base station 14 indication (sent by        mobile terminal 12S over feedback channel)/anchor_switching IE.        With this solution, in some embodiments, the        feedback_reporting_IE is used for both feeding bask C/I        information and for sending an anchor base station 14        identifier. Some indication needs to be generated either at the        base station 14 or mobile terminal 12 to convey that a given        feedback_reporting_IE will contain the anchor base station 14        identifier instead of the C/I information.

Option 5—monitor multiple base station 14s

-   -   Same as above 4 options with the exception that the mobile        terminal 12 monitors the MAPs from all member base stations 14        to know when and to which base station 14 to switch.    -   For MIMO blast soft handoff implementations, the solutions        provided above for macro-diversity soft handoff also apply.        UL Feedback Channel Switching

For Macro-Diversity Soft Handoff Implementations:

Option 1: The feedback channel assignment can be included in MOB-Bsofthandoff_REQ/RSP message synched with anchor base station 14 switchingand the real switching happens at the action time.

Option 2: If anchor_switching IE is defined, a new feedback channelassignment of new feedback channel can be included.

Option 3: At the first frame after action time, new anchor base station14 can assign a new feedback channel either through CHICH_Alloc_IE orfeedback channel subheader.

For Selective Soft Handoff Implementations:

Option 1: The feedback channel assignment can be included in MOB-Bsofthandoff_REQ/RSP message synched with anchor base station 14 switchingand the real switching happens at the action time.

Option 2: If anchor_switching IE is defined, a new feedback channelassignment of new feedback channel can be included.

Option 3: At the first frame after action time, new anchor base station14 can assign a new feedback channel either through CHICH_Alloc_IE orfeedback channel subheader.

For Fast Base Station Switching Implementations:

Option 1: The feedback channel assignment can be included in MOB-Bsofthandoff_REQ/RSP message synched with anchor base station 14 switchingand the real switching happens at the action time

Option 2: If anchor_switching IE is defined, a new feedback channelassignment of new feedback channel can be included

Option 3: At the first frame after action time, new anchor base station14 can assign a new feedback channel either through CHICH_Alloc_IE orfeedback channel subheader

For MIMO Blast Implementations:

Option 1: The feedback channel assignment can be included in MOB-Bsofthandoff_REQ/RSP message synched with anchor base station 14 switchingand the real switching happens at the action time.

Option 2: If anchor_switching IE is defined, a new feedback channelassignment of new feedback channel can be included.

Option 3: At the first frame after action time, new anchor base station14 can assign a new feedback channel either through CHICH_Alloc_IE orfeedback channel subheader.

UL Feedback Channel Reporting

For Macro-diversity soft handoff implementations, UL feedback channelreporting involves reporting an average C/I of all member base stations14.

For selective soft handoff implementations, UL feedback channelreporting involves reporting an average C/I of all member base stations14.

For MIMO blast implementations, UL feedback channel reporting involvesreporting an average C/I of all member base stations 14.

For fast base station switching, only the anchor base station's C/I isreported.

DL Data Transmission

For Macro-diversity soft handoff implementations, the mobile terminal12S monitors anchor base station 14. Only the anchor base station 14sends MAP (resource assignment). The mobile terminal 12S monitors onlythe anchor base station 14's control messages

For Selective Soft Handoff Implementations:

Option 1—the mobile terminal 12S monitors the anchor base station 14.Only the anchor base station 14 sends MAP include MAP_IE and Other basestation 14 IE(s)

Option 2—mobile terminals 12 monitor all member base stations' DL MAP.All transmitting base stations 14 (may be less than the active set ofbase stations 14) send their MAPs, and the mobile terminals 12 monitorall member base stations' MAPs.

For fast base station switching implementations the mobile terminal 12monitors only the anchor base station 14 after anchor base station 14switching.

For MIMO Blast Implementations:

Option 1—mobile terminal 12 monitors only anchor base station 14 (newbasic_DL_MIMO_other_base station 14 IE defined). Only anchor basestation 14 sends MAP include basic_MIMO_DL_MAP_IE and basic_MIMO_Otherbase station IE(s), and the mobile terminal 12 only monitors the anchorbase station 14.

Option 2—mobile terminal 12S monitors all members' MAPs. Alltransmitting base stations 14 (may be less than the active set of basestations 14) send their basic_MIMO_MAP les, and the mobile terminal 12monitors all member base stations' MAPs.

UL Data Transmission

For Macro-diversity soft handoff implementations, mobile terminal 12monitors anchor base station 14. Only the anchor base station 14 sendsMAP. The mobile terminal 12 monitors only the anchor base station'scontrol messages.

For selective soft handoff implementations mobile terminal 12 monitorsonly anchor base station 14, this being the same as macro-diversity softhandoff.

For fast base station switching implementations, mobile terminal 12monitors only the anchor base station 14 after anchor base stationswitching.

For MIMO blast implementations, mobile terminal 12 monitors only anchorbase station 14. Only the anchor base station 14 sends MAP includingbasic_MIMO_UL_MAP_IE, and mobile terminal 12 only monitors anchor basestation 14.

In some embodiments, a harmonized solution is provided. The solution isparticularly suited to, but not limited to, applications where theswitching frequency of anchor base station 14 (sending MAP andallocating UL resource for feedback channel of a mobile terminal 12 insoft handoff) in soft handoff is the same as that in FCS (fast basestation switching).

Harmonized Anchor Base Station Switching:

Option 1—Using MOB_MSsoft handoff_REQ/MOB_Bsoft handoff_RSP.

Option 2—define feedback channel reporting IE (poll one or multiplemobile terminal 12S to report their anchor base station 14 usingfeedback channel/anchor switching IE (indicate anchor switching andassigning new feedback channel).

Option 3: introduce a new dedicated UL anchor for base station 14switching. Define a reply with an anchor switching_IE from the basestation 14.

Harmonized DL Traffic Transmission Solution

The mobile terminal 12 only monitors the anchor base station 14 forDL/UL traffic process control commands. The anchor base station 14 sendscurrent DL DL MAP IE and may include other base station IEs as required.Detailed examples for each of the soft handoff formats will now beprovided:

Macro-Diversity Soft Handoff

-   -   Anchor sends DL MAP IE.    -   mobile terminal 12 monitors only anchor base station 14 and the        DL traffic process is transparent to mobile terminal 12 since        the other base station 14 will transmit using the same resources        and will appear as a multi-path component.

Selective Soft Handoff

-   -   Anchor sends DL MAP IE and Other base station 14 IE.    -   mobile terminal 12 monitor only the anchor base station 14 and        receives a MAP for both the Anchor and the other base station        14, and will understand that a selective combining is needed.

Fast Base Station Switching

-   -   Same as macro-diversity soft handoff.

MIMO Blast

-   -   Same as selective soft handoff.

Hybrid soft handoff and no soft handoff application—in some instancesone application may be delay-sensitive and require soft handoff whileanother application running on the same mobile terminal 12 is notdelay-sensitive and does not require soft handoff. A hybrid solution isprovided to deal with this by sending more than one MAP. In someembodiments, a bit is included to indicate soft handoff type. Forexample, soft handoff type=1 might indicate macro-diversity, and softhandoff type=0 might indicate selection diversity. Other values ofcourse may be used.

For macro-diversity, one MAP is sent for the real time application, andanother for the other application. For selective diversity, one MAP issent for the anchor base station 14 for the real time application, andanother for the second application. A third MAP is sent for the otherreal time base station 14, and selection diversity can be performed asbefore.

With reference to FIG. 11, a base station 14 configured according to oneembodiment of the present invention is illustrated. The base station 14generally includes a control system 20, a baseband processor 22,transmit circuitry 24, receive circuitry 26, multiple antennas 28, and anetwork interface 30. The receive circuitry 26 receives radio frequencysignals through antennas 28 bearing information from one or more remotetransmitters provided by mobile terminals 12. Preferably, a low noiseamplifier and a filter (not shown) cooperate to amplify and removebroadband interference from the signal for processing. Downconversionand digitization circuitry (not shown) will then downconvert thefiltered, received signal to an intermediate or baseband frequencysignal, which is then digitized into one or more digital streams.

The baseband processor 22 processes the digitized received signal toextract the information or data bits conveyed in the received signal.This processing typically comprises demodulation, decoding, and errorcorrection operations. As such, the baseband processor 22 is generallyimplemented in one or more digital signal processors (DSPs). Thereceived information is then sent across a wireless network via thenetwork interface 30 or transmitted to another mobile terminal 12serviced by the base station 14. The network interface 30 will typicallyinteract with the central network controller 16 and a circuit-switchednetwork forming a part of a wireless network, which may be coupled tothe public switched telephone network (PSTN).

On the transmit side, the baseband processor 22 receives digitized data,which may represent voice, data, or control information, from thenetwork interface 30 under the control of control system 20, and encodesthe data for transmission. The encoded data is output to the transmitcircuitry 24, where it is modulated by a carrier signal having a desiredtransmit frequency or frequencies. A power amplifier (not shown) willamplify the modulated carrier signal to a level appropriate fortransmission, and deliver the modulated carrier signal to the antennas28 through a matching network (not shown). The multiple antennas 28 andthe replicated transmit and receive circuitries 24, 26 provide spatialdiversity. Modulation and processing details are described in greaterdetail below.

With reference to FIG. 12, a mobile terminal 12 configured according toone embodiment of the present invention is illustrated. Similarly to thebase station 14, the mobile terminal 12 will include a control system32, a baseband processor 34, transmit circuitry 36, receive circuitry38, multiple antennas 40, and user interface circuitry 42. The receivecircuitry 38 receives radio frequency signals through antennas 40bearing information from one or more base stations 14. Preferably, a lownoise amplifier and a filter (not shown) cooperate to amplify and removebroadband interference from the signal for processing. Downconversionand digitization circuitry (not shown) will then downconvert thefiltered, received signal to an intermediate or baseband frequencysignal, which is then digitized into one or more digital streams. Thebaseband processor 34 processes the digitized received signal to extractthe information or data bits conveyed in the received signal. Thisprocessing typically comprises demodulation, decoding, and errorcorrection operations, as will be discussed in greater detail below. Thebaseband processor 34 is generally implemented in one or more digitalsignal processors (DSPs) and application specific integrated circuits(ASICs).

For transmission, the baseband processor 34 receives digitized data,which may represent voice, data, or control information, from thecontrol system 32, which it encodes for transmission. The encoded datais output to the transmit circuitry 36, where it is used by a modulatorto modulate a carrier signal that is at a desired transmit frequency orfrequencies. A power amplifier (not shown) will amplify the modulatedcarrier signal to a level appropriate for transmission, and deliver themodulated carrier signal to the antennas 40 through a matching network(not shown). The multiple antennas 40 and the replicated transmit andreceive circuitries 36, 38 provide spatial diversity. Modulation andprocessing details are described in greater detail below.

With reference to FIG. 13, a logical transmission architecture isprovided according to one embodiment. The transmission architecture isdescribed as being that of the base station 14, but those skilled in theart will recognize the applicability of the illustrated architecture forboth uplink and downlink communications. Further, the transmissionarchitecture is intended to represent a variety of multiple accessarchitectures, including, but not limited to code division multipleaccess (CDMA), frequency division multiple access (FDMA), time divisionmultiple access (TDMA), and orthogonal frequency division multiplexing(OFDM).

Initially, the central network controller 16 sends data (SDUs) intendedfor a mobile terminal 12 to the base station 14 for scheduling. Thescheduled data 44, which is a stream of bits, is scrambled in a mannerreducing the peak-to-average power ratio associated with the data usingdata scrambling logic 46. A cyclic redundancy check (CRC) for thescrambled data is determined and appended to the scrambled data usingCRC adding logic 48. Next, channel coding is performed using channelencoder logic 50 to effectively add redundancy to the data to facilitaterecovery and error correction at the mobile terminal 12. The channelencoder logic 50 uses known Turbo encoding techniques in one embodiment.

The resultant data bits are systematically mapped into correspondingsymbols depending on the chosen baseband modulation by mapping logic 52.Preferably, a form of Quadrature Amplitude Modulation (QAM) orQuadrature Phase Shift Key (QPSK) modulation is used. At this point,groups of bits have been mapped into symbols representing locations inan amplitude and phase constellation. Blocks of symbols are thenprocessed by space-time code (STC) encoder logic 54. The STC encoderlogic 54 will process the incoming symbols according to a selected STCencoding mode and provide N outputs corresponding to the number oftransmit antennas 28 for the base station 14. At this point, assume thesymbols for the N outputs are representative of the data to betransmitted and capable of being recovered by the mobile terminal 12.Further detail is provided in A. F. Naguib, N. Seshadri, and A. R.Calderbank, “Applications of space-time codes and interferencesuppression for high capacity and high data rate wireless systems,”Thirty-Second Asilomar Conference on Signals, Systems & Computers,Volume 2, pp. 1803-1810, 1998; R. van Nee, A. van Zelst and G. A.Atwater, “Maximum Likelihood Decoding in a Space Division MultiplexSystem”, IEEE VTC. 2000, pp. 6-10, Tokyo, Japan, May 2000; and P. W.Wolniansky et al., “V-BLAST: An Architecture for Realizing Very HighData Rates over the Rich-Scattering Wireless Channel,” Proc. IEEEISSSE-98, Pisa, Italy, Sep. 30, 1998 which are incorporated herein byreference in their entireties.

For illustration, assume the base station 14 has two antennas 28 (N=2)and the STC encoder logic 54 provides two output streams of symbols.Accordingly, each of the symbol streams output by the STC encoder logic54 is sent to a corresponding multiple access modulation function 56,illustrated separately for ease of understanding. Those skilled in theart will recognize that one or more processors may be used to providesuch analog or digital signal processing alone or in combination withother processing described herein. For example, the multiple accessmodulation function 56 in a CDMA function would provide the requisite PNcode multiplication, wherein an OFDM function would operate on therespective symbols using inverse discrete Fourier transform (IDFT) orlike processing to effect an Inverse Fourier Transform. Attention isdrawn to co-assigned application Ser. No. 10/104,399, filed Mar. 22,2002, entitled SOFT HANDOFF FOR OFDM, for additional OFDM details, andto RF Microelectronics by Behzad Razavi, 1998 for CDMA and othermultiple access technologies, both of which are incorporated herein byreference in their entireties.

Each of the resultant signals is up-converted in the digital domain toan intermediate frequency and converted to an analog signal via thecorresponding digital up-conversion (DUO) circuitry 58 anddigital-to-analog (D/A) conversion circuitry 60. The resultant analogsignals are then simultaneously modulated at the desired RF frequency,amplified, and transmitted via RF circuitry 62 and antennas 28. Notably,the transmitted data (PDUs) may be preceded by pilot signals, which areknown by the intended mobile terminal 12. The mobile terminal 12, whichis discussed in detail below, may use the pilot signals for channelestimation and interference suppression and the header foridentification of the base station 14.

Reference is now made to FIG. 14 to illustrate reception of thetransmitted signals by a mobile terminal 12. Upon arrival of thetransmitted signals at each of the antennas 40 of the mobile terminal12, the respective signals are demodulated and amplified bycorresponding RF circuitry 64. For the sake of conciseness and clarity,only one of the multiple receive paths in the receiver is described andillustrated in detail. Analog-to-digital (A/D) conversion anddownconversion circuitry (DCC) 66 digitizes and downconverts the analogsignal for digital processing. The resultant digitized signal may beused by automatic gain control circuitry (AGC) 68 to control the gain ofthe amplifiers in the RF circuitry 64 based on the received signallevel.

The digitized signal is also fed to synchronization circuitry 70 and amultiple access demodulation function 72, which will recover theincoming signal received at a corresponding antenna 40 at each receiverpath. The synchronization circuitry 70 facilitates alignment orcorrelation of the incoming signal with the multiple access demodulationfunction 72 to aid recovery of the incoming signal, which is provided toa signaling processing function 74 and channel estimation function 76.The signaling processing function 74 processes basic signaling andheader information to provide information sufficient to generate achannel quality measurement, which may bear on an overallsignal-to-noise ratio for the link, which takes into account channelconditions and/or signal-to-noise ratios for each receive path.

The channel estimation function 76 for each receive path provideschannel responses (h_(i,j)) corresponding to channel conditions for useby an STC decoder 78, if so desired or configured. The symbols from theincoming signal and channel estimates for each receive path are providedto the STC decoder 78, which provides STC decoding on each receive pathto recover the transmitted symbols. The channel estimates providesufficient channel response information to allow the STC decoder 78 todecode the symbols according to the STC encoding used by the basestation 14 and recover estimates corresponding to the transmitted bits.In a preferred embodiment, the STC decoder 78 implements MaximumLikelihood Decoding (MLD) for BLAST-based transmissions. As such, theoutputs of the STC decoder 78 are log likelihood ratios (LLRs) for eachof the transmitted bits, as will be described below in greater detail.These estimates, such as the LLRs, are then presented to channel decoderlogic 80 to recover the initially scrambled data and the CRC checksum.The channel decoder logic 80 will preferably use Turbo decoding.Accordingly, CRC logic 82 removes the CRC checksum, checks the scrambleddata in traditional fashion, and provides it to the de-scrambling logic84 for de-scrambling using the known base station de-scrambling code torecover the originally transmitted data 86.

Those skilled in the art will recognize improvements and modificationsto the preferred embodiments of the present invention. All suchimprovements and modifications are considered within the scope of theconcepts disclosed herein and the claims that follow.

What is claimed is:
 1. A method for facilitating a handoff for a mobileterminal in a wireless access network capable of supporting a pluralityof different handoff types, the method comprising: determining contextinformation associated with supporting wireless communications betweenthe wireless access network and the mobile terminal; and selecting ahandoff type from the plurality of handoff types based on whether thecontext information is shared between base stations involved in ahandoff or transferred from one base station to another of the basestations involved in the handoff, wherein the selecting a handoff typeis performed by at least one of a group comprising the mobile terminal,one of the base stations involved in the handoff, the wireless accessnetwork, or a central network controller.
 2. The method of claim 1wherein at least one of the plurality of handoff types is fast basestation switching.
 3. The method of claim 1 wherein at least one of theplurality of handoff types is soft handoff.
 4. The method of claim 3wherein the soft handoff is macro-diversity soft handoff.
 5. The methodof claim 3 wherein the soft handoff is selective soft handoff.
 6. Themethod of claim 1 wherein at least two of the plurality of handoff typesare fast base station switching and soft handoff.
 7. The method of claim1 wherein the context information is associated with a plurality oflevels, and selecting a handoff is further based on a level associatedwith the context information.
 8. The method of claim 7 wherein the levelassociated with the context information corresponds to service-relatedinformation for a communication session.
 9. The method of claim 7wherein the level associated with the context information corresponds tolayer 2 processing related information provided at the base stationsinvolved in the handoff.
 10. The method of claim 7 wherein the pluralityof levels comprises four levels, each of the four levels representingdifferent levels of information associated with wireless communications.11. The method of claim 1 wherein selecting a handoff is further basedon the content of the context information.
 12. The method of claim 1wherein selecting a handoff is further based on an applicationassociated with a communication session.
 13. The method of claim 1wherein selecting a handoff is further based on channel conditions. 14.The method of claim 1 further comprising determining if one or more ofthe base stations involved in the handoff supports the plurality ofhandoff types.
 15. The method of claim 1 further comprising determiningif the mobile terminal supports the plurality of handoff types.
 16. Themethod of claim 1 wherein when the context information is not shared ortransferred, the method further comprising selecting a hard handoff. 17.The method of claim 1 wherein when either the mobile terminal or thebase stations involved in the handoff do not support one of theplurality of handoff types, the method further comprising selecting ahard handoff.
 18. The method of claim 1 wherein one of the base stationsinvolved in the handoff selects the handoff type.
 19. The method ofclaim 1 wherein the mobile terminal selects the handoff type.
 20. Anapparatus for facilitating a handoff for a mobile terminal in a wirelessaccess network capable of supporting a plurality of different handofftypes, the apparatus comprising: at least one communication interface;and a control system associated with the at least one communicationinterface adapted to: determine context information associated withsupporting wireless communications between the wireless access networkand the mobile terminal; and select a handoff type from the plurality ofhandoff types based on whether the context information is shared betweenbase stations involved in a handoff or transferred from one base stationto another of the base stations involved in the handoff.